That is All:

First of all, their parallels. Both geniuses, top of their field. Both suffered an accident that physically changed them, forever, and not in a wholesome Spider-Man kind of way. Both try to do what they can to help others despite their own issues; Banner heals people, Tony works on developing clean energy. And both struggle, in their own way, with duality; Tony and Iron Man, Bruce and the Hulk. Two identities, one body. Only difference is Iron Man’s bad side is Tony.

Agreed. I love how the movie portrays the relationship between the two. Kudos to Joss Whedon for an incredibly fun, entertaining and exciting film.